I would stay away from willow plants as they suck up water everywhere. I also would recommend look like others have mentioned try to think about introducing Nitrogen fixing trees/bushes or plants every 3 or 4th tree.
A big idea is once you can bring in animals who will help add nutrients, etc. Possibly start with a couple of chickens. At some point you need to bring in animals.
Gabe Brown's notes:
1. Use the least amount of mechanical or chemical disturbance possible — Keep tilling to a bare minimum and strive to avoid all synthetic fertilizers, pesticides, herbicides and fungicides.
2. Armor the soil surface with living plants — Forest and prairie land is completely covered with vegetation, and this is the environment farmers need to emulate. That vegetation protects the soil not only from wind and water erosion, but also from excessive heating and cooling. These living plants are what end up actually "growing" topsoil.
3. Have living roots in the soil as long as possible — This is an extension of armoring with cover crops. Soil is formed from growing plants that take in carbon dioxide (CO2) from the atmosphere through photosynthesis, and release it as "liquid carbon" through the roots, depositing it back into the soil, where it attracts microorganisms that ultimately end up providing the plant with all the nutrients it needs to grow.
4. Diversify — Having a diverse array of plant life is essential, and cover crops fulfill this requirement as well. Cover crops, helping to improve the soil, attract beneficial insects and capture more sunlight (energy).
5. Integrate livestock and other animals, including insects — Flowering plants that attract pollinators and predator insects will naturally help ward off pests that might otherwise decimate your main crop.